Climates of the 20th and 21st Centuries Simulatedby NCAR Climate System Model

Dai, A., T.M.L Wigley, B.A. Boville, J.T. Kiehl, and L.E.
Buja

J. Climate, submitted in Oct.1999, revised
in March 2000

Abstract

The Climate System Model (CSM), a coupled global climate
model without``flux adjustments'' recently developed at NCAR, was
used to simulate the20th century climate using historical greenhouse gas
and sulfate aerosolforcing. This simulation was extended through the 21st
century under twonewly developed scenarios, a business-as-usual case (ACACIA-BAU,CO2 ~710 ppmv in 2100) and a CO2 stabilization case (STA550,
CO2 ~540ppmv in 2100). Here we compare the simulated and observed
20th centuryclimate, and then describe the simulated climates for
the 21st century. Themodel simulates the spatial and temporal variations of
the 20th century climatereasonably well. These include the rapid rise in global-
and zonal-meansurface temperatures since the late 1970s, the precipitation
increases overnorthern middle- and high-latitude land areas, ENSO-induced
precipitationanomalies, and pole-midlatitude oscillations (such as
the NAO) in sea-levelpressure fields. The model has a cold bias (2-6°C)
in surface air temperatureover land, overestimates of cloudiness (by 10-30%) over
land, and underestimatesof marine stratus clouds to the west of North and South
America and Africa.

The projected global surface warming from the 1990s to
the 2090s is ~1.9°Cunder the BAU scenario and ~1.5°C under the STA550
scenario. In bothcases, the middle stratosphere cools due to the increase
in CO2, whereas thelower stratosphere warms in response to recovery of the
ozone layer. As in othercoupled models, the surface warming is largest at winter
high latitudes (>5.0°Cfrom the 1990s to the 2090s) and smallest (~1.0°C)
over the southern oceans,and is larger over land areas than ocean areas. Globally
averaged precipitationincreases by ~3.5% (3.0%) from the 1990s to the 2090s
in the BAU (STA) case.In the BAU case, large precipitation increases (up to
50%) occur over northernmid- and high-latitudes and over India and the Aradian
Peninsula. Markeddifferences occur between the BAU and STA regional precipitation
changesresulting from inter-decadal variability. Surface evaporation
increases at alllatitudes except for 60-90°S. Water vapor from increased
tropical evaporationis transported into mid- and high-latitudes and returned
to the surface throughincreased precipitation there. Changes in soil moisture
content are small (within ±3%).Total cloud cover changes little, although there is a
upward shift of middle-levelclouds. Surface diurnal temperature range decreases by
about 0.2-0.5°C over mostland areas. 2-8 day synoptic storm activity decreases
(by up to 10%) at lowlatitudes and over midlatitude oceans, but increases
over Eurasia and Canada.The cores of subtropical jets move slightly up- and equator-ward.
Associatedwith reduced latitudinal temperature gradients over mid-
and high-latitudes, thewintertime Ferrel cell weakens (by 10-15%). The Hadley
circulation also weakens(by ~10%), partly due to the upward shift of cloudiness
which produces enhancedwarming in the upper troposphere.